Method of fabricating a merged P-N junction and schottky diode with regrown gallium nitride layer

ABSTRACT

A method for fabricating a merged p-i-n Schottky (MPS) diode in gallium nitride (GaN) based materials includes providing an n-type GaN-based substrate having a first surface and a second surface. The method also includes forming an n-type GaN-based epitaxial layer coupled to the first surface of the n-type GaN-based substrate, and forming a p-type GaN-based epitaxial layer coupled to the n-type GaN-based epitaxial layer. The method further includes removing portions of the p-type GaN-based epitaxial layer to form a plurality of dopant sources, and regrowing a GaN-based epitaxial layer including n-type material in regions overlying portions of the n-type GaN-based epitaxial layer, and p-type material in regions overlying the plurality of dopant sources. The method also includes forming a first metallic structure electrically coupled to the regrown GaN-based epitaxial layer.

This application claims priority from co-pending U.S. Provisional PatentApplication No. 13/866,286, filed Apr. 19, 2013, entitled “METHOD OFFABRICATING A MERGED P-N JUNCTION AND SCHOTTKY DIODE WITH REGROWNGALLIUM NITRIDE LAYER,” which is hereby incorporated by reference, as ifset forth in full in this document, for all purposes.

BACKGROUND OF THE INVENTION

Power electronics are widely used in a variety of applications. Powerelectronic devices are commonly used in circuits to modify the form ofelectrical energy, for example, from AC to DC, from one voltage level toanother, or in some other way. Such devices can operate over a widerange of power levels, from milliwatts in mobile devices to hundreds ofmegawatts in a high voltage power transmission system. Despite theprogress made in power electronics, there is a need in the art forimproved electronics systems and methods of operating the same.

SUMMARY OF THE INVENTION

The present invention relates generally to electronic devices. Morespecifically, the present invention relates to forming merged p-i-n andSchottky (MPS) diodes using III-nitride semiconductor materials. In someembodiments, merged p-n and Schottky diodes are provided, which can alsobe referred to as MPS diodes. Merely by way of example, the inventionhas been applied to methods and systems for manufacturing MPS diodesusing a regrowth of gallium-nitride (GaN) based epitaxial layers thatinclude both p-type and n-type regions. These MPS diodes can be used ina range of applications that can benefit from a high-voltage switch withlow capacitance and very low leakage current in the off state.

MPS diodes utilize a device structure that can be designed to exhibitthe low turn-on voltage of a Schottky diode in the forward direction,and the low reverse leakage current of a p-n diode in the reverse. Inaddition, GaN MPS diodes outperform competitors in Si and SiC materialsystems due to the outstanding material properties of GaN. As describedherein, an MPS diode includes a Schottky contact with embedded p-type(e.g., p+) regions within the area of the contact. The forward turn-onis dominated by the Schottky portion before the p-n junction turns on ata higher voltage. The reverse mode of operation is dominated byappropriately spaced p-n junctions. The same processing steps used toform the p-type regions may also be utilized as components of edgetermination of the device.

According to an embodiment of the present invention, a method forfabricating a merged p-i-n Schottky (MPS) diode in gallium nitride (GaN)based materials includes providing an n-type GaN-based substrate havinga first surface and a second surface. The method also includes formingan n-type GaN-based epitaxial layer coupled to the first surface of then-type GaN-based substrate, and forming a p-type GaN-based epitaxiallayer coupled to the n-type GaN-based epitaxial layer. The methodfurther includes removing portions of the p-type GaN-based epitaxiallayer to form a plurality of dopant sources, and regrowing a GaN-basedepitaxial layer including n-type material in regions overlying portionsof the n-type GaN-based epitaxial layer, and p-type material in regionsoverlying the plurality of dopant sources. The method also includesforming a first metallic structure electrically coupled to the regrownGaN-based epitaxial layer.

According to another embodiment of the present invention, an MPS diodecan include a III-nitride substrate having a first side and a secondside opposing the first side. The III-nitride substrate is characterizedby a first conductivity type. The MPS diode also includes a firstIII-nitride epitaxial layer coupled to the III-nitride substrate andcharacterized by the first conductivity type, and a plurality of dopantsources coupled to the first III-nitride epitaxial layer andcharacterized by a second conductivity type. The MPS diode furtherincludes a second III-nitride epitaxial layer overlying portions of thefirst III-nitride epitaxial layer and overlying the plurality of dopantsources. The second III-nitride epitaxial layer includes first regionscharacterized by the first conductivity type and second regionscharacterized by the second conductivity type. The MPS diode alsoincludes a first metallic structure electrically coupled to the secondIII-nitride epitaxial layer.

According to yet another embodiment of the present invention, an MPSdiode can include a III-nitride substrate, an n-type III-nitrideepitaxial layer coupled to the III-nitride substrate, and a plurality ofp-type dopant sources coupled to the n-type III-nitride epitaxial layer,each of the p-type dopant sources having opposing etched surfaces. TheMPS diode can further include a regrown III-nitride epitaxial layercomprising n-type regions coupled to portions of the n-type III-nitrideepitaxial layer and the opposing etched surfaces, and p-type regionscoupled to the plurality of p-type dopant sources and interspersedbetween the n-type regions. A bulk interface is present between adjacentn-type and p-type regions.

Numerous benefits are achieved by way of the present invention overconventional techniques. For devices fabricated using embodiments of thepresent invention, utilization of the combined material properties ofGaN such as high critical electric field, high electron mobility, andhigh thermal conductivity result in devices with performance advantages.The MPS diodes described herein have lower leakage currents for largereverse voltages compared to standard Schottky diodes, and almost nopenalty in forward turn-on voltage. At large reverse bias conditions,the depletion region resulting from one reverse p-n junction will extendand merge with neighboring p-n junctions. In this case, the Schottkymetal-to-semiconductor junction will not experience a large reverseelectrical field, thus producing less leakage current. The device issuitable for applications where Schottky diodes are currently used,including high voltage applications, since the properties of GaN includehigh critical electric field at breakdown.

The MPS diode retains the fast switching speed of a majority carrierSchottky diode as long as the forward voltage does not exceed the level(e.g. about 3 volts) at which the p-n junction portion of the deviceturns on and injects minority carriers into the drift region, which needto be swept out when switching from on to off, as in a typical p-i-ndiode. The regime when the p-i-n diode turns on is outside the normalarea of operation for the device, but serves a useful purpose for surgecurrent conditions. The minority carrier injection, which is detrimentalin terms of switching speed, provides a benefit of lower on-stateresistance than a Schottky diode would have due to conductivitymodulation of the drift region.

The GaN MPS diode described herein provides a high voltage switch withexcellent tradeoff between blocking voltage and forward resistance. TheMPS diode has very low leakage current in the off state. A low forwardresistance allows for a smaller area diode for a given current rating.Since capacitance scales with area, such a diode will retain excellentswitching characteristics due to its low capacitance. The processing andfabrication methods described herein can also provide an effective edgetermination technique suitable for commercial applications of the GaNMPS diode since edge termination enables the diode to reach the fullpotential associated with the outstanding GaN material properties. Theseand other embodiments of the invention, along with many of itsadvantages and features, are described in more detail in conjunctionwith the text below and attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-6 are simplified cross-sectional diagrams illustrating a processflow for fabrication of an MPS diode according to an embodiment of thepresent invention.

FIG. 7 is a simplified flowchart illustrating a method of fabricating anMPS diode according to an embodiment of the present invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Embodiments of the present invention relate to electronic devices. Morespecifically, the present invention relates to forming merged p-i-nSchottky (MPS) diodes using III-nitride semiconductor materials. Merelyby way of example, the invention has been applied to methods and systemsfor manufacturing MPS diodes using gallium-nitride (GaN) based epitaxiallayers. These MPS diodes can be used in a range of applications that canbenefit from a high-voltage switch with low capacitance and very lowleakage current in the off state. Additional description related to MPSdiodes is provided in U.S. patent application Ser. No. 13/270,625,entitled “Method of Fabricating a GaN Merged P-I-N Schottky (MPS)Diode,” filed on Oct. 10, 2011, and U.S. patent application Ser. No.13/585,121, entitled “Method of Fabricating a Gallium Nitride MergedP-I-N Schottky)MPS) Diode by Regrowth and Etchback,” filed on Aug. 14,2012, the disclosures of which are hereby incorporated by reference intheir entirety for all purposes.

GaN-based electronic and optoelectronic devices are undergoing rapiddevelopment, and are expected to outperform competitors in silicon (Si)and silicon carbide (SiC). Desirable properties associated with GaN andrelated alloys and heterostructures include high bandgap energy forvisible and ultraviolet light emission, favorable transport properties(e.g., high electron mobility and saturation velocity), a high breakdownfield, and high thermal conductivity. In particular, electron mobility,μ, is higher than competing materials for a given doping level, N. Thisprovides low resistivity, ρ, because resistivity is inverselyproportional to electron mobility, as provided by equation (1):

$\begin{matrix}{{\rho = \frac{1}{q\;\mu\; N}},} & (1)\end{matrix}$where q is the elementary charge.

Another superior property provided by GaN materials, includinghomoepitaxial GaN layers on bulk GaN substrates, is high criticalelectric field for avalanche breakdown. A high critical electric fieldallows a larger voltage to be supported over smaller length, L, than amaterial with a lower critical electric field. A smaller length forcurrent to flow together with low resistivity give rise to a lowerresistance, R, than other materials, since resistance can be determinedby equation (2):

$\begin{matrix}{{R = \frac{\rho\; L}{A}},} & (2)\end{matrix}$where A is the cross-sectional area of the channel or current path.

Homoepitaxial GaN layers on bulk GaN substrates also have relatively lowdefect density compared to materials grown on mismatched substrates,such as GaN grown on silicon, silicon carbide (SiC), or sapphire.Homoepitaxial GaN layers on bulk GaN substrates therefore have largeminority carrier lifetime in intrinsic and/or low-doped regions ofsemiconductor devices that use these materials, enhancing the carrierinjection effect for wider base regions. The low defect density alsogives rise to superior thermal conductivity.

As described herein, MPS diodes created using homoepitaxial GaN layersnot only are able to take advantage of the outstanding physicalqualities of these materials, but also benefit from the structuraladvantages provided by combining p-i-n and Schottky diodes. An MPS diodeis a device structure that can be designed to exhibit the low turn-onvoltage of a Schottky diode in the forward direction, and the lowreverse leakage current of a p-i-n diode in the reverse direction. AnMPS diode can include a Schottky contact with one or more embedded p-njunctions (or p-i-n regions) within the area of the contact. The forwardturn-on voltage is dominated by the Schottky portion before the p-njunctions turn on at a higher forward voltage. The reverse mode ofoperation is dominated by appropriately spaced p-n junctions.Furthermore, in some embodiments, the same processing used to form thep-n junctions embedded within the area of the Schottky contact also canbe used to form edge termination structures to provide edge terminationfor the MPS diode.

According to embodiments of the present invention, gallium nitride (GaN)epitaxy on bulk or pseudo-bulk GaN substrates is utilized to fabricateMPS diodes and/or edge termination structures not possible usingconventional techniques. For example, conventional methods of growingGaN include using a foreign substrate such as SiC. This can limit thethickness of a usable GaN layer grown on the foreign substrate due todifferences in thermal expansion coefficients and lattice constantbetween the GaN layer and the foreign substrate. High defect densitiesat the interface between GaN and the foreign substrate furthercomplicate attempts to create edge termination structures for varioustypes of semiconductor devices.

FIGS. 1-6 are simplified cross-sectional diagrams illustrating a processflow for fabrication of an MPS diode according to a first embodiment ofthe present invention.

FIG. 1 illustrates an epitaxial system suitable for use with embodimentsof the present invention. As illustrated in FIG. 1, a first III-Nepitaxial layer 110 (e.g., a GaN lightly doped n-type drift layer) isformed on a III-N substrate 100 (e.g., an n-type GaN substrate) havingthe same conductivity type. The III-N substrate 100 will be referred toas a GaN substrate below and can be a bulk or pseudo-bulk GaN materialon which the first GaN epitaxial layer 110 is grown. Dopantconcentrations (e.g., doping density) of the GaN substrate 100 can vary,depending on desired functionality. For example, GaN substrate 100 canhave an n+ conductivity type, with dopant concentrations ranging from1×10¹⁷ cm⁻³ to 1×10²⁰ cm⁻³. Although the GaN substrate 100 isillustrated as including a single material composition, multiple layerscan be provided as part of the substrate. Moreover, adhesion, buffer,and other layers (not illustrated) can be utilized during the epitaxialgrowth process. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

Although some embodiments provided herein (e.g., the embodimentillustrated in FIGS. 1-6) are discussed in terms of GaN substrates andGaN epitaxial layers, the present invention is not limited to theseparticular binary III-V materials and is applicable to a broader classof III-V materials, in particular III-nitride materials. Additionally,although a GaN substrate is illustrated in some implementations,embodiments of the present invention are not limited to GaN substrates.Other III-V materials, in particular, III-nitride materials, areincluded within the scope of the present invention and can besubstituted not only for the illustrated GaN substrate, but also forother GaN-based layers and structures described herein. As examples,binary III-V (e.g., III-nitride) materials, ternary III-V (e.g.,III-nitride) materials such as InGaN and AlGaN, quaternary III-nitridematerials, such as AlInGaN, doped versions of these materials, and thelike are included within the scope of the present invention.

The properties of the first III-N epitaxial layer 110, referred to as aIII-N epitaxial layer or a first GaN epitaxial layer below, can alsovary, depending on desired functionality. The first GaN epitaxial layer110 can serve as a drift layer for the Schottky region(s) of the MPSdiode and an intrinsic component for the p-i-n junction(s) of the MPSdiode. Thus, the first GaN epitaxial layer 110 can be a relativelylow-doped material. For example, the first GaN epitaxial layer 110 canhave an n− conductivity type, with dopant concentrations ranging from1×10¹⁴ cm⁻³ to 1×10¹⁸ cm⁻³. Furthermore, the dopant concentration can beuniform, or can vary, for example, as a function of the thickness of thedrift region.

The thickness of the first GaN epitaxial layer 110 can also varysubstantially, depending on the desired functionality. As discussedabove, homoepitaxial growth can enable the first GaN epitaxial layer 110to be grown far thicker than layers formed using conventional methods.In general, in some embodiments, thicknesses can vary between 0.5 μm and100 μm, for example. In other embodiments thicknesses are greater than 5μm. Resulting breakdown voltages for the MPS diode can vary depending onthe embodiment. Some embodiments provide for breakdown voltages of atleast 100V, 300V, 600V, 1.2 kV, 1.7 kV, 3.3 kV, 5.5 kV, 13 kV, or 20 kV.

Different dopants can be used to create n- and p-type GaN epitaxiallayers and structures disclosed herein. For example, n-type dopants caninclude silicon, oxygen, germanium, or the like. P-type dopants caninclude magnesium, beryllium, zinc, or the like.

FIG. 2 illustrates the formation, for example, by epitaxial growth, of ap-type III-N epitaxial layer 210. As described below, the p-type III-Nepitaxial layer 210 (e.g., a GaN layer) will provide dopant sourceregions fabricated from the epitaxial material. The thickness of theepitaxial layer 210 can range from about 50 nm to about 1000 nm in someembodiments, for example, 100 nm. In some particular embodiments, adopant species (e.g., magnesium) is formed on the first GaN epitaxiallayer 110, with the thickness of the epitaxial layer 210 tending towardszero. In these particular embodiments, the dopant source is only orsubstantially the dopant species. In other embodiments, a thin epitaxiallayer (e.g., ranging from 1 nm to 100 nm) that is heavily doped withp-type dopants is grown to form the layer used in providing the dopantsources. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

FIG. 3 illustrates the formation of dopant sources 310 a and 310 b bythe patterning and removal of portions of the p-type III-N epitaxiallayer 210. The removal of portions of the p-type III-N epitaxial layerto form the dopant sources 310 a and 310 b can be performed by acontrolled etch using an etch mask (not shown but having the dimensionsof openings 305 between adjacent dopant sources). The vertical sides ofthe dopant sources can be referred to as opposing etched surfaces. Asdescribed more fully below, epitaxial structures forming part of theSchottky portions of the MPS diode are formed in the openings 305, sothe removal of portions of the p-type III-N epitaxial layer can bedesigned to stop at approximately the top surface 320 of the first GaNepitaxial layer 110 or at a desired depth in the first GaN epitaxiallayer. Moreover, the removal process may involve multiple etching stepsresulting in different depths for different components of the MPS diode.Inductively-coupled plasma (ICP) etching and/or other common GaN etchingprocesses can be used. One of ordinary skill in the art would recognizemany variations, modifications, and alternatives.

FIG. 4 illustrates epitaxial regrowth according to an embodiment of thepresent invention. As described below, the regrown GaN-based epitaxiallayer 410 includes n-type material in regions overlying portions of then-type GaN-based epitaxial layer 110 and p-type material in regionsoverlying the plurality of dopant sources 310. As illustrated in FIG. 4,a III-N epitaxial layer 410 has been regrown, filling the openings 305and extending to a height greater than the height of the dopant sources310 a and 310 b, for example, a height ranging from about 50 nm to about500 nm. In a particular embodiment, the thickness of III-N epitaxiallayer 410 is about 200 nm. The III-N epitaxial layer 410 can be referredto as a second epitaxial layer or a regrown epitaxial layer sinceepitaxial layer 410 is coupled to portions of the first epitaxial layer110 and regrown over the dopant sources 310. The presence of the dopantsources results in III-N epitaxial layer 410 having differing dopantcharacteristics as a function of lateral position. In regions 405 a, 405b, and 405 c, n-type material (e.g., n− GaN epitaxial material) isregrown. Accordingly, in these regions, an n-type region is formed inelectrical connection with the n-type drift layer.

In regions 415 a and 415 b, the presence of the dopant sourcesunderlying the regrown material results in p-type doping of regions 415a and 415 b. Thus, as stated above, the dopant characteristics varylaterally, with p-type regions interspersed among n-type regions.Without limiting embodiments of the present invention, the inventorsbelieve that p-type dopants (e.g., magnesium) present in the dopantsources carry over into the regrown material, producing p-type dopant(e.g., magnesium) incorporation into the regrown material. Magnesiumpresent on the top surface of the dopant sources can be incorporated inthe regrown material although this particular mechanism is not requiredby embodiments of the present invention. In some embodiments, althoughthe regrowth conditions during the formation of III-N epitaxial layer410 are suitable for the growth of n-type material, resulting in n-typeregions 405 a, 405 b, and 405 c, p-type regions 415 a and 415 b are alsoformed, despite the presence of n-type dopants in the regrowthprecursors.

Referring to FIG. 4, p-n junctions are present at the interface betweenn-type layer 110 and dopant sources 310 a and 310 b, which are, in turnelectrically connected to p-type regions 415 a and 415 b. N-type regionssuitable for the Schottky diode elements are provided by n-type regions405 a, 405 b, and 405 c in electrical contact with n-type layer 110.Although not illustrated, embodiments of the present invention canutilize interfacial layers before or during the regrowth process. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

One method of regrowing epitaxial layer 410 can be through a regrowthprocess as described more fully in U.S. patent application Ser. No.13/198,666, entitled “Method and System for Formation of p-n Junctionsin Gallium Nitride Based Electronics,” filed on Aug. 4, 2011, thedisclosure of which is hereby incorporated by reference in its entirety.As illustrated herein, blanket regrowth provides benefits in comparisonwith some selective area regrowth techniques since in some selectiveregrowth techniques, the hard mask can decompose, resulting inincorporation of the hard mask materials into the epitaxially regrownmaterials. By using a blanket regrowth process, the regrowth mask iseliminated and material properties are improved.

Referring once again to FIG. 4, the regrown regions 415 a and 415 bdisposed between regrown regions 405 a, 405 b, and 405 c arecharacterized by a different conductivity type than the first GaNepitaxial layer 110, thereby forming the p-i-n structures of the MPSdiode. In one embodiment, for example, the regrown material in regions415 a and 415 b has a p+ conductivity type and the first GaN epitaxiallayer 110 has an n− conductivity type. The dopant concentration of theregions can be relatively high, for example in a range from about 1×10¹⁷cm⁻³ to about 2×10²⁰ cm⁻³. In some embodiments, the dopant concentrationin regrown regions 415 a and 415 b decreases as a function of distancefrom dopant sources 310 a and 310 b, respectively.

The thickness of the regrown epitaxial layer 410 can vary, depending onthe process used to form the layer and the device design. In someembodiments, the thickness of the epitaxial layer 410 is between 0.05 μmand 5 μm, for example, 0.2 μm.

The dopant concentration of regrown epitaxial regions 405 can be uniformor non-uniform as a function of thickness, depending on desiredfunctionality. In some embodiments, for example, the dopantconcentration of n-type dopants in regions 405 increases or decreaseswith thickness, such that the dopant concentration is relatively low orhigh near the n-type GaN drift layer 110 and increases or decreases asthe distance from the drift layer increases.

Referring to FIG. 4, in an embodiment, the p-type doping concentrationin regions 415 at surface 440 will be higher than the dopingconcentration at surface 442 as the doping concentration decreases withheight. For the n-type doping in regions 405, the doping concentrationat surface 444 can be lower or higher than the doping concentration atsurface 446 in some embodiments as the doping decreases or increaseswith height, although this is not required by embodiments of the presentinvention. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

Embodiments of the present invention provide benefits not availableusing conventional techniques since the interface 450 between the p-typeand n-type materials is a bulk interface formed during the epitaxialregrowth process, not an etched interface that would be present if thep-type material was regrown adjacent to as-grown n-type material thathad been etched to define the lateral characteristics of the MPS device.Although the bulk interfaces 450 are illustrated as vertical in thefigure, this is not required by the present invention and otherinterface profiles including curved surfaces are included within thescope of the invention. Referring to FIG. 4, the height of interface 450is typically greater than the thickness of the dopant sources, resultingin MPS diodes in which portions (e.g., the majority) of the thickness ofthe regrown n-type epitaxial material includes substantially verticalp-n interfaces in bulk material. The inventors have determined that p-ninterfaces in bulk material provide enhanced device properties incomparison to p-n interfaces present at etched surfaces such as surface451.

As illustrated in FIG. 4, the MPS diode includes a plurality of P-Njunction regions 405 that include a regrown p-type material 415, anepitaxial p-type material 310, and n-type material (drift layer 110).Interspersed with the p-n junction regions, the MPS diode has aplurality of Schottky regions that include regrown n-type material 405and n-type material (drift layer 110). The interfaces between theregrown p-type regions and the regrown n-type regions include (e.g., aresubstantially) bulk epitaxial interfaces in addition to etchedinterfaces between the dopant sources and the regrown n-type regions.Thus, in a single regrowth process, both p-type and n-type regions arefabricated in an epitaxially regrown layer simultaneously.

Although embodiments of the present invention are discussed in relationMPS diodes and methods for fabricating MPS diodes, the invention is notlimited to this particular device structure and can be applied moregenerally to lateral non-uniform doping of III-nitride materials and thefabrication of devices that utilize lateral non-uniform doping. Thus, avariety of GaN-based electronic devices in which a regrown GaN layerincludes interspersed n-type and p-type regions are included within thescope of the present invention. These devices can include a lateral p-njunction structure that includes a substrate, n-type epitaxially grownregions, p-type epitaxially grown regions, and regrown GaN-basedlayer(s) that include both n-type regrown material and p-type regrownmaterial. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives.

FIG. 5A illustrates the formation of ohmic and Schottky contacts on thesurface of the regrown III-N layer 410. Ohmic contacts 510 a and 510 bare made to p-type regrown regions 415 a and 415 b, respectively andSchottky contacts 520 are made to n-type regrown regions 405, with asingle Schottky contact to region 405 b illustrated in FIG. 5A. Althoughseparate ohmic and Schottky contacts are illustrated in FIG. 5A, this isnot required by embodiments of the present invention and a singlecontact metal structure can be formed using one or more layers of metaland/or alloys designed to create a Schottky barrier with the n-typeregrown regions 405, which have a relatively low dopant concentrationand ohmic contacts with p-type regrown regions 415, which haverelatively high dopant concentration and form the p-i-n portions of theMPS diode. Although not illustrated for purposes of clarity, peripheralp-type regrown regions can be utilized for edge termination of the MPSdiode. The contact metals can be formed using a variety of techniques,including lift-off and/or deposition with subsequent etching, which canvary depending on the metals used. In some embodiments, the contactmetals can include nickel, platinum, palladium, silver, gold, scandiumand the like.

FIG. 5B illustrates an alternative embodiment in which ohmic contactsare formed as discussed in relation to FIG. 5A followed by formation ofblanket Schottky contact 550.

FIG. 6 illustrates the formation of a back metal structure 610 on thebottom of GaN substrate 100. The back metal structure 610 can be one ormore layers of ohmic metal that serve as a contact for the cathode ofthe MPS diode. For example, the back metal structure 610 can comprise atitanium-aluminum (Ti/Al) ohmic metal. Other metals and/or alloys can beused including, but not limited to, titanium, aluminum, nickel,palladium, gold, silver, combinations thereof, or the like. In someembodiments, an outermost metal of the back metal structure 610 caninclude gold, tantalum, tungsten, palladium, silver, or aluminum,combinations thereof, and the like. The back metal structure 610 can beformed using any of a variety of methods such as sputtering,evaporation, or the like.

FIG. 7 is a simplified flowchart illustrating a method of fabricating anMPS diode (i.e., a merged p-i-n Schottky diode or a merged p-n Schottkydiode) according to an embodiment of the present invention. The method700 is suitable for the fabrication of an MPS diode in gallium nitride(GaN) based materials. The method includes providing an n-type GaN-basedsubstrate having a first surface and a second surface (710) and formingan n-type GaN-based epitaxial layer coupled to the first surface of then-type GaN-based substrate (712). The n-type GaN-based epitaxial layercan be used as the drift layer of the MPS diode. The n-type GaN-basedsubstrate is characterized by a first n-type dopant concentration andthe n-type GaN-based epitaxial layer is characterized by a second n-typedopant concentration that is typically less than the first n-type dopantconcentration.

The method also includes forming a p-type GaN-based epitaxial layercoupled to the n-type GaN-based epitaxial layer (714) and removingportions of the p-type GaN-based epitaxial layer to form a plurality ofdopant sources (716). The method further includes regrowing a GaN-basedepitaxial layer including n-type material in regions overlying portionsof the n-type GaN-based epitaxial layer and p-type material in regionsoverlying the plurality of dopant sources (718). In someimplementations, the regrowth of the GaN-based epitaxial layer is ablanket regrowth process.

Additionally, the method includes forming a first metallic structureelectrically coupled to the regrown GaN-based epitaxial layer (720). Inan embodiment, the first metallic structure includes Schottky contactselectrically coupled to the n-type material in the regions overlyingportions of the n-type GaN-based epitaxial layer and ohmic contactselectrically coupled to the p-type material in the regions overlying theplurality of dopant sources. In order to fabricate the cathode of theMPS diode, a second metallic structure electrically coupled to thesecond surface of the n-type GaN-based substrate can be formed. Inaddition to diode functionality, embodiments of the present inventioncan also include edge termination functionality in which at least one ofthe regions overlying the plurality of dopant sources is configured toprovide edge termination to the MPS diode. Additionally, at least one ofthe regions overlying the plurality of dopant sources can be configuredto provide a junction termination extension to the MPS diode.

It should be appreciated that the specific steps illustrated in FIG. 7provide a particular method of fabricating an MPS diode according to anembodiment of the present invention. Other sequences of steps may alsobe performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 7 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this applicationand scope of the appended claims.

What is claimed is:
 1. A method for fabricating a merged p-i-n Schottky(MPS) diode in gallium nitride (GaN) based materials, the methodcomprising: providing an n-type GaN-based substrate having a firstsurface and a second surface; forming an n-type GaN-based epitaxiallayer coupled to the first surface of the n-type GaN-based substrate;forming a p-type GaN-based epitaxial layer coupled to the n-typeGaN-based epitaxial layer; removing portions of the p-type GaN-basedepitaxial layer to form a plurality of dopant sources; regrowing aGaN-based epitaxial layer including n-type material in regions overlyingportions of the n-type GaN-based epitaxial layer and p-type material inregions overlying the plurality of dopant sources; and forming a firstmetallic structure electrically coupled to the regrown GaN-basedepitaxial layer.
 2. The method of claim 1 wherein the first metallicstructure comprises Schottky contacts electrically coupled to the n-typematerial in the regions overlying portions of the n-type GaN-basedepitaxial layer and ohmic contacts electrically coupled to the p-typematerial in the regions overlying the plurality of dopant sources. 3.The method of claim 1 further comprising forming a second metallicstructure electrically coupled to the second surface of the n-typeGaN-based substrate.
 4. The method of claim 1 wherein regrowing theGaN-based epitaxial layer comprises using a blanket regrowth process. 5.The method of claim 1 wherein the n-type GaN-based substrate ischaracterized by a first n-type dopant concentration and the n-typeGaN-based epitaxial layer is characterized by a second n-type dopantconcentration less than the first n-type dopant concentration.
 6. Themethod of claim 1 wherein at least one of the regions overlying theplurality of dopant sources is configured to provide edge termination tothe MPS diode.
 7. The method of claim 1 wherein at least one of theregions overlying the plurality of dopant sources is configured toprovide a junction termination extension to the MPS diode.